As super user:

iptables -I INPUT -s 1.2.3.4 -j DROP

If you ever take a look at your server logs, you know there are tens of thousands of consecutive failed login attempts from various IP addresses. Even though the chances of success of these brute force login attempts is rather low, why gamble?

A quick and easy way to address this problem is BlockHosts. It is a simple Python script which will scan logs and block IPs based on a configurable number of failed login attempts and time thresholds. Cron the script to your taste.

http://www.aczoom.com/cms/blockhosts